Photovoltaic inverter is the energy conversion device in a photovoltaic power generation system, with which direct current (DC) generated by the photovoltaic cells is converted into alternative current (AC) and then sent into the power grid. The topological structure diagram of most low-medium power photovoltaic inverters is shown in FIG. 1, including an inverting circuit and multiple DC chopper circuits connected in parallel, where multiple photovoltaic strings are connected to the DC bus via the multiple DC chopper circuits and outputted via the inverting circuit.
Under the circumstance that the bus voltage meets inversion requirements, the inverter control system stops some DC chopper circuits with a relatively low transformation ratio from working so that the photovoltaic strings, which are connected with these DC chopper circuits, are connected directly in parallel to the DC bus to improve conversion efficiency of the whole system; afterwards the inverter control system tracks the overall maximum power point of the photovoltaic strings (which are directly connected in parallel with the DC bus via DC chopper circuits stopping operating) as a whole by adjusting the voltage of the DC bus to improve photoelectric conversion efficiency of the photovoltaic strings, which is defined as the Maximum Power Point Tracking (MPPT) centralized modes of the photovoltaic inverter.
In practice, when illumination intensity changes, maximum power points of each of the photovoltaic strings will deviate from the overall maximum power point of the photovoltaic strings, which will result in a decrease in MPPT efficiency of the photovoltaic inverter so as to be unable to achieve both a high conversion efficiency of the whole system and a high MPPT efficiency.